A rotor of a rotating electrical machine having a cylindrical rotor core provided to the outer circumference of spider arms is well known, e.g., in JP-A No. 2008-61356 and JP-A No. 2010-4583.
FIG. 5 shows a structure of a rotor of a well known rotating electrical machine in JP-A No. 2008-61356 and JP-A No. 2010-4583. This figure shows an axial top view (lower) and its partial enlarged view (upper) of a vertical axis water turbine generator. A rotating shaft 2 is at the center of the top view. Multiple spider arms 6 extend radially from the rotating shaft 2 in the radial direction. A rotor core 7 is secured via the multiple spider arms 6.
The rotor core 7 of components forming the rotor of the rotating electrical machine of the vertical axis water turbine generator is a circular steel plate formed by arranging uniformly shaped fan shaped thin segment cores circumferentially, which are laminated axially while being offset circumferentially to structure the cylindrical rotor core. Key ways are provided to the inner circumference of the segment cores at the same pitch as the circumferentially offset pitch. Accordingly, as shown in the partial enlarged view of FIG. 5, multiple key ways 11a are formed to the inner circumference of the cylindrically structured rotor core 7 at aligned positions.
On the other hand, a key way 10 is formed also to the radial top end of each spider arm 6 extending radially from the vertical rotating shaft 2 in the radial direction. The key way 11a of the rotor core 7 is disposed correspondingly to the key way 10 of the radial top end of each spider arm 6.
The spider arms 6 and the rotor core 7 are joined by inserting a T key 14 into the key ways (10 and 11a) and then inserting cotters 15 into circumferential gaps between the T key 14 and key ways (10 and 11a). Accordingly, the rotor core 7 is secured to the spider arms 6.
Specifically, for example, the head of the T key 14 is positioned in the key way 10 of the spider arm and the foot of the T key 14 is positioned in the key way 11a of the rotor core. When the foot of T key 14 is merely inserted into key way 11a of the rotor core, the gaps are produced. Therefore, cotters 15a and 15b are inserted into the gaps on both sides of this foot.
According to this engagement structure, there is no gap between the T key 14 and key way 11a during the shutdown of the rotating electrical machine, but the rotor rotates when the rotating electrical machine operates to apply a stress to the rotor core 7 due to centrifugal force. Then, the rotor core 7 displaces to widen circumferentially and radially. Particularly, the circumferential expansion becomes large because the stress gathers around the periphery of the key way 11a. Then, circumferential gaps are produced between the T key and key way 11a. 
During the electrical generation by the rotating electrical machine, torque from a water turbine (not shown) directly connected to the vertical rotating shaft 2 is applied to the head of the T key 14 of the top end of each spider arm 6 and is transmitted to the rotor core 7 via the cotters 15 from the foot of the T key 14. As a result, each spider arm 6 is pushed onto the key way 11a in the rotational direction for electrical generation, and a resultant force F of pushing forces f is applied as a restraining force of the rotor core 7 on the spider arms 6.
That is, as shown by an arrow H, when the rotational direction for electrical generation is right in FIG. 5, the spider arms 6 and rotor core 7 engage on the right side (the side of the cotter 15b) of the key way 11a to apply the pushing force. Then, a circumferential gap 19 is produced on the left side (side of the cotter 15a) between the T key 14 and key way 11a. 
On the other hand, a force to make the rotor core 7 eccentric by rotation, eccentric load P, applied on the rotor core 7 due to its own unbalance. Sufficient balancing can make the eccentric load P of the rotor core 7 smaller than the restraining force F by the push of the T key 14. Therefore, even when the gap 19 is produced circumferentially between the T key 14 and key way 11a during the electrical generation, the rotor core 7 is not offset eccentrically to the spider arms 6.
A water turbine generator may perform a pumping operation. In this case, the rotational direction is reverse. The spider arms 6 and rotor core 7 engage on the right side (side of the cotter 15b) of the key way 11a to apply the pushing force because the rotor core 7 is a source of torque generation. That is, also in the water turbine, irrespective of the mode of operation generator, the pushing force is applied by the engagement on the right side.
By the way, when a load cut off and an emergency stop arise during electrical generation, a rotational speed rises temporarily to enter a dangerous situation. Thus, it is necessary to stop a water turbine by closing an inlet valve of the water turbine. In such a state, in an early over speed state, the gap 19 between the T key 14 and key way 11a becomes larger. After that, when the acceleration begins to decrease, the pushing force of the T key 14 by the torque from the water turbine decreases. Then, when the rotational speed reaches near the maximum to close the inlet valve of the water turbine, the reverse torque is applied on the T key 14 for deceleration in turn. The application direction of the pushing force f of the T key 14 is reversed.
That is, at the early stage of the load cut off, the spider arms 6 and rotor core 7 engage on the right side of the key way 11a to apply the pushing force and then to produce the gap 19 circumferentially on the left side of the key way 11a between the T key 14 and key way 11a. After that, the engagement occurs on the left side of the key way 11a. Further, this intermediate state is to be called a floating state, in which the active engagement is not achieved on either side.
FIG. 6 shows relationship of the forces applied on each component at the time of the load cut off. The rotational direction H is maintained for electrical generation, but the pushing force f onto the right side of the key way 11a is reduced due to the large deceleration force.
In this process, the eccentric load P of the rotor core 7 becomes larger than the restraining force F by the push of the T key 14. The rotor core 7 is offset in the range of the gap between the T key 14 and key way 11a to be eccentric to the spider arms 6. Accordingly, the center of gravity of the rotor core 7 is displaced relative to the center of gravity of the spider arms 6. Then, an axial runout of the rotating shaft increases by an amplification ratio or over at its rotational speed. This may cause damage of a machine.
It is desirable to provide a rotating electrical machine in which a rotor core is not eccentric to spider arms even when a torque from a water turbine is reduced and a reverse torque is applied due to, e.g., load cut off.